This invention relates to video signal recording and, more particularly, to a method and apparatus for recording compressed audio data along with the video data on a record medium, such as a digital video tape.
Digital signal recording apparatus, such as digital video tape recorders (DVTR) have been developed for broadcasting purposes because of the excellent quality in video pictures that are reproduced therefrom. Heretofore, the emphasis of such digital video recording systems has been on enhancing the video picture. Typically, analog video signals are digitized by sampling the analog signal and converting each sample to an 8-bit digital signal. Maximum video information is retained by recording the 8-bit video samples in uncompressed form. Consequently, a substantial amount of record medium is needed to record video programs of even minimal broadcasting length, such as thirty minute programs.
DVTR systems have been developed with two distinct formats: the so-called component type digital recorder, known as the D-1 format, and the so-called composite type digital recorder, known as the D-2 format. When recording digital video signals on video tape using the D-1 format, an audio recording sector is provided in the middle of a track and is preceded and followed by video recording sectors. Up to four channels of audio information may be recorded in the audio sector. By providing the audio sector in the middle of the track, it is expected that the usual scanning heads will exhibit stable track following, or tracking control, by the time those heads reach the middle of the track during normal as well as special effect scanning speeds. It has been found that small scanning errors that may be present when the video sectors are scanned may be more easily corrected and are less perceptible to a human observer than if those same small scanning errors are present when audio information is scanned. By locating the audio information in the middle of a track, such small scanning errors that may be present when audio information is reproduced are minimized.
When video and audio information are recorded in the D-2 format, the audio information is recorded in audio sectors that precede and follow the video sector. The video sector is, of course, substantially larger than each of the audio sectors, and two channels of audio information may be recorded in each audio sector. Thus, a total of four channels of audio information may be recorded in the D-2 format, with two channels of audio information preceding the video sector and two channels of audio information following the video sector. The track length of the D-2 format is shorter than the track length of the D-1 format by approximately 13%. As a result of this shorter track length, tracking control errors are minimized over the entire length of the track and, thus, the flanking audio sectors are scanned quite stably even when different tape speeds are used to effect different reproducing modes (e.g. normal, slow speed, high speed and special effects modes).
The four-channel audio recording capability of the D-1 and D-2 formats facilitates the recording of audio information with high quality. If audio information normally is recorded as two-channel audio data (e.g. left and right audio channels), then the same two-channel audio data may be recorded twice so as to be recorded as four-channel audio data. Such redundant recording is most helpful in overcoming or correcting errors that otherwise would result from drop-out in one or the other pairs of audio channels. Four-channel audio recording also facilitates a technique known as "after-recording" in which audio information 6 that is produced at a later time (i.e. subsequent to the recording of video information) may be recorded. For example, when audio information is edited or when audio information is translated into a different language, such edited or translated audio data usually is provided at a time that is substantially delayed from the time that the video information is recorded. Nevertheless, this after-recorded audio data is recorded quite easily in two of the four audio channels that are available for recording.
As mentioned above, digital video recording for broadcasting purposes is accomplished without compression of the video or audio data. While this assures a high quality of video information, the fact that the video data is uncompressed means that a substantial amount of record medium must be used to accommodate such uncompressed video data. Since video tapes must be of finite length, the need for more record medium to record uncompressed video data results in a shorter overall recording time. This inefficient use of video tape is an acceptable trade-off for broadcasting purposes but is not acceptable for consumer use. A desirable objective of video tape recording is the capability of recording a broadcasted program in the user's absence. Since a consumer may use his video recorder to record several programs, it is important that commercially available "blank" magnetic tape (such as in cassette form) be of sufficient length to record the many programs that the user otherwise may miss. Hence, for consumer use, a desirable objective is to record video and audio data on a single record medium over a long period of time.
To accommodate long recording periods, digital video recorders for consumer use have been developed to record a channel of audio data at a sampling rate of 48 KHz, with each audio sample being digitized as a 16-bit sample. Such consumer-type digital video recorders typically are capable of recording only two audio channels; and, as a result, if errors are present in the recorded audio information, it is difficult to correct such errors. Thus, it is difficult to obtain high quality audio recording in such two-channel consumer-type video recorders. Furthermore, the typical consumer-type two-channel audio recording capability does not easily permit the after-recording of audio information.